The subject matter disclosed herein relates generally to touch sensitive systems, and more particularly to determining whether a touch is being held on a bending wave touch panel of a touch sensitive system.
Touch panels are used to provide two-dimensional coordinate information. One example may be an opaque track pad while another example may be a transparent touchscreen placed in front of a display such as a liquid crystal display. Touch panels may be based on a variety of touch technologies including four-wire and five-wire resistive, capacitive, infrared and surface acoustic wave types, as well as bending wave touch technologies.
Bending waves may also be referred to as “flexural waves” as well as “lowest order anti-symmetric Lamb waves”. Bending waves in a plate, such as a touchscreen substrate, are characterized by motion that is largely perpendicular to the surface and essentially the same at all depths with respect to the surface. Bending wave touch systems may use frequencies typically within the kilohertz (kHz) range.
For example, bending wave touch systems may detect a touch based on a tap of an object, such as a stylus or finger, used to excite bending waves in a substrate. The bending waves induce electrical signals in piezoelectric elements or sensors (piezos) bonded to the substrate. The electrical signals are captured by electronics and processed to determine (X,Y) coordinates of the touch position, such as by using time-of-flight methods to extract touch coordinate information from piezo signals. In other systems, an “acoustic fingerprint” may be determined based on the electrical signals and then compared with a library of acoustic fingerprints or templates having known (X,Y) coordinates to identify the best matching template. Acoustic fingerprints may be constructed as frequency profiles of signal amplitudes, signal amplitude ratios, signal phases, and/or phase differences between signals. Phase difference profiles have been found to be particularly useful acoustic fingerprints. Other bending wave systems may use other methods to identify the (X,Y) coordinates. Elo TouchSystems, a business unit of Tyco Electronics, offers acoustic fingerprint based bending-wave touch systems under the trade name “APR” or “Acoustic Pulse Recognition”.
None of the conventional bending wave touch systems, however, can identify a hold condition, wherein the user has touched the touchscreen and is holding the finger or other object at the same (X,Y) coordinate location. Unless the user is tapping, dragging or otherwise moving the finger on the surface, bending wave signals in the kilohertz range are not generated and the system does not know that the finger is present on the surface. During a hold, the system may falsely assume that a lift-off has occurred as no (X,Y) coordinates are being detected.